Modern computer systems typically utilize high-density packaging techniques which enable many components to be mounted on a printed circuit (PC) board. The components are typically mounted to the surfaces of the PC board and are interconnected via multiple signal and power conductors in layers internal to the board. These surface-mounted components are thereafter connected to components mounted on other boards of the system by signal wires of a system bus, thereby resulting in a significant number of signal connections throughout the system. For a high speed computer system using, e.g., emitter-coupled logic (ECL) technology, these signal connections are terminated at nodes, i.e., test nodes, usually having a uniform resistance value. A variation from that value may be an indication of loss of signal integrity.
During production of the computer system, an audit of the resistances at these test nodes is typically performed by a technician to insure proper design and manufacture of the system. Because of the high-density packaging techniques employed with the multi-layer PC boards, short (and open) circuit connections internal to the boards and between components mounted on the boards are difficult to detect visually. Accordingly, the technician checking the system must examine every test node to verify proper electrical signal connections throughout the system. This typically includes checking each test node of a fully-populated PC board for a particular resistance value. Such testing is known as "in-circuit" testing. For example, a short-circuit connection between two signal wires, each of which is terminated with a 50.OMEGA. resistor, presents a resistance value of 25.OMEGA. at a test node for either wire, instead of the expected 50.OMEGA. value.
One tool that is commonly used to measure resistances is an ohmmeter. The ohmmeter is a simple instrument that applies a fixed voltage across two resistors in series, e.g., a resistor of known value and a resistor whose value is being measured. The current through the known resistor is measured by a voltmeter, which is calibrated in terms of resistance values and has a display scale that indicates such values. However, visual inspection of the scale can prove cumbersome and time-consuming when the technician is auditing a significant number of test nodes in a computer system.
Another tool used to measure resistances is a conventional, multi-purpose volt-ohm-millimeter (VOM) meter, i.e., "multimeter". Although the multimeter features a "continuity" setting that beeps when a short-circuit or very small ohmic value is detected, it is not configured to detect any arbitrary value of resistance.
U.S. Pat. No. 3,784,906 discloses a multiple null bridge circuit with test leads for determining whether the value of a resistor is within a predetermined tolerance range. The bridge circuit includes high-impedance amplifiers to reduce bridge loading. However, the open-circuit voltage at the test leads is approximately 10 volts; this is unacceptable for most in-circuit testing applications because such a voltage level may be sufficient to activate or even damage certain active components on the PC board.
In addition, this bridge circuit includes a visual indicator interface to display the category of a manufactured resistor during trimming operations. Specifically, a "steady-state" illumination of the "GO" lamp indicates a desired resistance value. If the measurement of a "HIGH" resistance value immediately precedes measurement of a "LOW" value, all three indicators are illuminated prior to settling at the desired "LOW" state, thus requiring prolonged observance of the lamps.
Therefore, it is among the objects of the present invention to provide a resistance testing device that produces an audible signal upon detection of a predetermined resistance value.
Another object of the present invention is to provide a portable resistance testing device that is adapted for "in-circuit" testing applications.